It has come to my attention that a crisis has developed over petrol, so far there has been no strike by fuel delivery drivers but Francis Maude has stirred up some panic by suggesting that the public should fill up their cars and put some petrol to one side in their homes.
I would like to remind my readers not to handle petrol near any of the following.
1. Gas cookers or other devices which use flames
2. Electrical equipment, this includes light switches. Never turn a light switch on or off when there is a gas leak
3. Anything else which makes sparks
I have seen an interesting quote from a fire fighter (Matt Wrack ), he said. “Petrol is highly flammable, highly explosive, easily ignited and toxic”
Now lets go through this statement one part at a time.
Petrol has a very low flash point, the flash point is the lowest temperature at which the fumes above a combustible liquid will ignite when a flame is presented to them. A highly flammable liquid has a flash point of 32 degrees C or lower, as typical petrol (motor car fuel) has a flash point below zero degrees C it counts as a highly flammable liquid.
The firefighter is quotes as saying that petrol is explosive, I would disagree. Liquid petrol will not detonate in the same way as TNT can so it is not a true explosive. But mixtures of air and petrol can ignite and deflagrate. A deflagration is an event where a flame front travels through a material, a deflagration can emit a lot of heat and even create an overpreassure. Without high speed photography it can be hard to distinguish between deflagration and detonation.
So while I think that the fire fighter is wrong to label petrol as an explosive, I would say that a clear violent reaction hazard exists with petrol.
In some ways a deflagration (or fuel air explosion) with petrol can give a higher energy yield than many explosives. For example TNT is very oxygen poor and tends to form a lot of soot when it detonates. But the same mass of petrol when mixed in a suitable way with air can release more energy but I would say that I would expect that the “explosion” will be a slower and slightly more gentle event.
The autoignition temperature of petrol is an important thing, the autoignition temperature is the lowest temperature of a surface which is able to ignite a mixture of air and fuel. This is a measure of how easy it is to ignite a fuel.
I have found a table of autoignition temperatures, petrol ignites at only 280 oC, while acetone (nail varnish remover) needs 465 oC and toluene (a paint thinner) needs 535 oC to ignite it. So the fireman is right to say that petrol is easy to ignite.
The fireman stated that petrol was toxic, I would say that “toxic” can be hard to judge. While petrol is no where near the worst toxic substance I can think of (just trust me I can think of some real nasties) I would say that exposure to petrol can lead to some horrible health effects. So petrol has to be at least “harmful” and it can have some toxic effects (like giving petrol sniffers brain damage) so I think we will agree with the fireman.